Study On The Effect Of Lactic Acid Bacteria With High Yield Of Exopolysaccharides On Blueberry Juice Fermentation

Blueberry fruit is sweet and delicate,rich in nutrients and phenolic substances including dietary fiber and other nutrients.Lactic acid bacteria(LAB)fermentation can effectively enhance and improve the nutritional value and flavor of blueberry juice with has probiotic effects.At present,the researches on fermented blueberry juice are mostly based on the number of viable bacteria,and there are few reports on the content of exocytopolysaccharide(EPS)in blueberry juice.There is a lack of lactic acid bacteria strains with high EPS production which are suitable for blueberry juice,and the changes of phenolic compounds in blueberry juice during the fermentation of LAB are not clear.This study aimed to screen strains with strong EPS production ability from 70 strains of LAB,study the probiotic characteristics of the screened strains,identify 3 strains and mix them to ferment blueberry juice,optimize the process of EPS production of fermented blueberry juice,analyze the changes of physicochemical indexes of blueberry juice during storage periods,and study the effects of fermentation on phenolic compounds in blueberry juice,which layed the foundation for the development of functional beverages and was of great significance for improving product quality.The results of this study were as follows:(1)Taking EPS content as an index,3 LAB strains with high yield of exopolysaccharides were screened by phenol-sulfuric acid method,namely R2,9sh and SR2-6.According to morphological observation,physiological and biochemical tests and16 S r DNA gene analysis,the identification results of 3 LAB strains were as follows: strain R2 was Leuconostoc mesenteroides,strain 9sh was Lactocacillus plantarum,and strain SR2-6 was Lactocacillus fermentum.The probiotic characteristics of 3 LAB strains were studied by measuring acid resistance,bile salt resistance,α-glucosidase inhibition ability,α-amylase inhibition ability,artificial gastrointestinal fluid external simulated digestion,and polyphenolic compound transformation.The results show that: strains 9sh and SR2-6 had strong acid resistance,strains R2 and SR2-6 had strong resistance to bile salts,strain 9sh had the ability of inhibition α-amylase activity,strains R2 and SR2-6 had the ability of inhibition α-glucosidase activity,and three LAB strains had gastrointestinal fluid tolerance and the ability of polyphenol compound conversion.Based on comprehensive consideration,strains R2,9sh and SR2-6 were selected for mixed fermentation.(2)The optimal fermentation process conditions were obtained by response surface method and artificial neural network,which was the ratio 2:1:1 of strains Lactiplantibacillus plantarum 9sh,Lactocacillus fermentum SR2-6 and Leuconostoc mesenteroides R2,6% lactose,0.6% soybean peptide,initial pH 4.5,8% inoculation amount,fermentation temperature at 30 °C,fermentation time in 60 h.The maximum EPS content was obtained under these conditions as 3.537 g/L.(3)The number of viable bacteria,pH value,EPS,total phenols,total anthocyanins and soluble solids of fermented blueberry juice decreased with the increase of storage time and temperature(P < 0.05).The total acid content increased with the increase of storage time and temperature(P < 0.05).There was no significant change in total flavonoid content during storage periods(P > 0.05).The pH,total anthocyanins and total flavonoids of sterilized fermented blueberry juice decreased with the increase of storage time and temperature.Total acids,EPS and soluble solids did not change significantly during storage periods(P > 0.05).Therefore,low temperatures helped preserve fermented blueberry juice,delay the loss of nutrients,and extend shelf life.(4)The effects of lactic acid bacteria fermentation on phenolic compounds in blueberry juice were analyzed by metabolomics.The results showed that a total of 726 polyphenol metabolites were detected in fermented blueberry juice,including 44.77%flavonoids,41.32% phenolic acids,11.98% lignin and coumarin,0.83% tannin and 1.1%others,of which 42 metabolites were significantly upregulated,33 metabolites were significantly down-regulated,and 651 metabolites were not significantly different.Significantly different metabolites were involved in KEGG metabolic pathways which included biosynthesis of secondary metabolites,tyrosine acid metabolism,,phenylpropane biosynthesisand other metabolic pathways.Three significantly enriched metabolic pathways were involved,namely tyrosine metabolism,phenylalanine metabolism and other metabolic pathways.